Additive compositions capable of changing the viscosity of various liquids are of considerable interest for a number of commercial and industrial applications. Dense CO2, such as supercritical carbon dioxide (scCO2), is of particular interest as a solvent in chemical processing. scCO2 is non-flammable, relatively non-toxic, and naturally abundant. Similarly, as naturally occurring elements of natural gas, natural gas liquids (NGLs) are also of interest. High pressure hydrocarbon mixtures (for example, methane, ethane, propane, butane, and pentane) are commonly injected after primary production and many years of waterflooding to enhance oil production from subterranean layers of porous sandstone or carbonate rock.
Current fracturing and oil recovery techniques employ large amounts of water and/or scCO2 to extract oil from geological formations. Aqueous fracturing and extraction methods are most common and require significant treatment and/or disposal of large quantities of produced water. Fracturing and oil recovery methods based on scCO2 have several advantages including higher solubility of oil in scCO2, which increases extraction efficiency, as well as a substantial decrease in the amount of aqueous waste which needs to be treated. Unfortunately the low viscosity of scCO2 (˜20× less than water) results in fingering, where the scCO2 tends to find the path of least resistance resulting in poor overall oil recovery. The low viscosity of dense CO2 as a fracturing fluid results in small fractures and difficulty in the transport of proppant particles into the fractures.
While it has been recognized that a dense CO2 thickener could be a desirable solution for reducing the mobility of dense CO2, thickening dense CO2 is a difficult task as dense CO2 is a poor solvent for most organic thickener molecules. Additives, such as high molecular weight polymers and associating polymers, have been synthesized. However, such products did not achieve both solubility in dense CO2, in the absence of high volume of co-solvents, and thickening of dense CO2 by altering the viscosity of dense CO2.
Accordingly, there is a need to develop viscosifiers of dense CO2 and NGLs, which could be used to improve oil recovery performance and/or reduce the need to treat or dispose of large quantities of produced water.